To reduce the risk of damage to the drill bit in consequence of limited control of the exact position of and strain on the drill bit in deep boreholes, it is known to use solutions pertaining to the design of the drill bit itself. Some of the solutions aim at making the drill bit less aggressive and thereby less sensitive when meeting a work surface against which the drill bit is brought. An example of one such solution used is the reduction of a so-called angle of attack of the cutting elements of the drill bit, so that less reactive force is produced in relation to weight-on-bit. Weight-on-bit is a known term in the drilling industry and relates to the axial force exerted by the drill bit against the work surface. With reduced reactive force, a lower risk of vibrations in the radial direction of the drill bit is achieved. In the specialist environment, such vibrations are known as “stick-slip” which is a stop-and-start motion or “jerking”. The same object is sought to be achieved by producing the cutting elements of the drill bit with a bevel instead of a straight cutter angle.
Generally, the drawback of such solutions is that a considerable increase in weight-on-bit is required when the cutters are to establish engagement in hard rock. Such an increased weight-on-bit is exponentially reflected in increased frictional heat in the cutting elements of the drill bit, which constitutes a technological barrier; the cutting elements that are used in a drill bit for drilling a subsea formation or a rock on land, are made, in the main, from polycrystalline or synthetic diamond in which the diamond fragments are bonded together into circular plates by the use of cobalt. This material combination is sensitive to heat, and the strength is reduced exponentially from 350° C. In reality, said increased frictional heat in the cutting elements of the drill bit causes existing solutions, with protection of the drill bit when being fed in, to be incompatible with drilling in bedrock, for example. This is a considerable barrier to economic utilization of geothermal heat and deep oil and gas deposits.
Consequently, optimum utilization of such sharp, energy-efficient cutting elements that are necessary for cutting hard rock without overheating must be based on principles other than reduced angle of attack and increased supporting surface. The prior art is common, axial shock absorbers for a drill string. Such shock absorbers went through a rapid development in the period from around 1960 to around 1980 and gained considerable ground together with roller bits.
From this period, the following publications are known: U.S. Pat. Nos. 3,073,134, 3,225,566, 3,329,221, 3,382,936, 3,947,008, 3,963,228, 4,054,040, 4,133,516, 4,162,619, 4,173,130, 4,186,569, 4,194,582, 4,210,316, 4,211,290, 4,257,245, 4,303,138, 4,398,898, 3,871,193 and 4,901,806.
Out of the above-mentioned publications, the publication U.S. Pat. No. 4,186,569 is particularly interesting, as it discloses an axial shock absorber to be built into a drill string with the object of preventing axial vibrations and shocks during drilling. The object is achieved by using a telescopic unit with straight, axial splines to transmit torque, the unit being kept extended by means of springs. Oil is used as the damping medium in accordance with a known principle. Of particular interest in this device is a separate counterspring, the purpose of which is to balance the force from internal pressure and stretching from the gravitational force acting from the part of the drill string that is arranged below the unit in the direction of the drill bit when the drill bit is free, above the bottom, that is, or has low weight-on-bit.
However, axial shock absorbers were phased out with the introduction of drill bits with shearing cutters as these have insignificant vibration challenges in the axial direction, but all the more risk from impacts or jerking in the radial direction. Such impacts or jerking may occur especially when the drill bit is being fed into the borehole and at transition zones between rocks or formations of different characters, typically at the transition from one rock to a subsequent harder rock.
From the publication US 20140090892, an apparatus for maintaining so-called “weight-on-bit”, or “thrust” which is the axial force exerted by the drill bit against the work surface during a drilling operation, is known. The apparatus is a rotationally rigid damping device arranged to damp axial vibrations in a drill string.
A more suited solution for reducing said impacts or jerking is a torque converter of the kind that transforms undesired impacts and “peaks” in the torque into a mechanically controlled axial motion which proportionally relieves the drill bit. However, this solution requires rigid internal compression springs. This rigidity makes the torque converter give little or no protection at low strains, for example in the engagement phase. That is to say, as the drill bit is brought into engagement with a rock.
The latter mechanical torque converters for drill strings were developed somewhat later than axial shock absorbers for drill bits. Such mechanical torque converters are known from the publications U.S. Pat. No. 7,044,240 and NO 315209.
Said publications NO 315209 and U.S. Pat. No. 7,044,240 disclose torque converters for building into a drill string with the object of preventing overload from torque variation during drilling. The object is achieved by using a telescopic unit with an internal, steep thread coupling, wherein the unit is kept extended or expanded by means of great cooperating forces provided by means of a compression spring and internal fluid pressure and the gravitational force acting on the part of the drill string that is arranged below the unit, in the direction of the drill bit, that is. Undesired impacts and torque peaks are converted through the threaded coupling into an axial contraction proportionally relieving the drill bit. The expansion forces mentioned above have as their purpose to restore the axial force on the drill bit as soon as the torque load decreases. A person skilled in the art will know that such expansion forces must be large. However, large expansion forces have the disadvantageous effect of the torque converter being fully extended and in practice rigid during feeding against the work surface. Consequently, the solutions that are disclosed in NO 315209 and U.S. Pat. No. 7,044,240 have a very limited effect before the drill bit has become fully engaged. This means that in cases in which the work surface is uneven, the drill bit may be damaged before the torque converter gets into function. To reduce or avoid said disadvantageous effect, tests have been performed with a reduction in the expansion forces acting on the drill bit, to bring the torque converter into function at a lower load, accordingly to provide a more “sensitive” torque converter. However, said tests have shown that such a more “sensitive” torque converter gives a reduced capacity during normal operation, drilling, that is.
From the publication US 2014262650, an apparatus for damping torsional oscillations to which a drill string may become subjected in a well, is known. The apparatus is axially rigid.